/*
 *
 *  * Copyright (C) 2015 Square, Inc.
 *  *
 *  * Licensed under the Apache License, Version 2.0 (the "License");
 *  * you may not use this file except in compliance with the License.
 *  * You may obtain a copy of the License at
 *  *
 *  *      http://www.apache.org/licenses/LICENSE-2.0
 *  *
 *  * Unless required by applicable law or agreed to in writing, software
 *  * distributed under the License is distributed on an "AS IS" BASIS,
 *  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  * See the License for the specific language governing permissions and
 *  * limitations under the License.
 *
 */
package com.lark.oapi.okio;

import javax.annotation.Nullable;
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import java.io.IOException;
import java.security.InvalidKeyException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

import static com.lark.oapi.okio.Util.checkOffsetAndCount;

/**
 * A sink that computes a hash of the full stream of bytes it has accepted. To use, create an
 * instance with your preferred hash algorithm. Write all of the data to the sink and then call
 * {@link #hash()} to compute the final hash value.
 *
 * <p>In this example we use {@code HashingSink} with a {@link BufferedSink} to make writing to the
 * sink easier. <pre>   {@code
 *
 *   HashingSink hashingSink = HashingSink.sha256(s);
 *   BufferedSink bufferedSink = Okio.buffer(hashingSink);
 *
 *   ... // Write to bufferedSink and either flush or close it.
 *
 *   ByteString hash = hashingSink.hash();
 * }</pre>
 */
public final class HashingSink extends ForwardingSink {

    private final @Nullable
    MessageDigest messageDigest;
    private final @Nullable
    Mac mac;

    private HashingSink(Sink sink, String algorithm) {
        super(sink);
        try {
            this.messageDigest = MessageDigest.getInstance(algorithm);
            this.mac = null;
        } catch (NoSuchAlgorithmException e) {
            throw new AssertionError();
        }
    }

    private HashingSink(Sink sink, ByteString key, String algorithm) {
        super(sink);
        try {
            this.mac = Mac.getInstance(algorithm);
            this.mac.init(new SecretKeySpec(key.toByteArray(), algorithm));
            this.messageDigest = null;
        } catch (NoSuchAlgorithmException e) {
            throw new AssertionError();
        } catch (InvalidKeyException e) {
            throw new IllegalArgumentException(e);
        }
    }

    /**
     * Returns a sink that uses the obsolete MD5 hash algorithm to produce 128-bit hashes.
     */
    public static HashingSink md5(Sink sink) {
        return new HashingSink(sink, "MD5");
    }

    /**
     * Returns a sink that uses the obsolete SHA-1 hash algorithm to produce 160-bit hashes.
     */
    public static HashingSink sha1(Sink sink) {
        return new HashingSink(sink, "SHA-1");
    }

    /**
     * Returns a sink that uses the SHA-256 hash algorithm to produce 256-bit hashes.
     */
    public static HashingSink sha256(Sink sink) {
        return new HashingSink(sink, "SHA-256");
    }

    /**
     * Returns a sink that uses the SHA-512 hash algorithm to produce 512-bit hashes.
     */
    public static HashingSink sha512(Sink sink) {
        return new HashingSink(sink, "SHA-512");
    }

    /**
     * Returns a sink that uses the obsolete SHA-1 HMAC algorithm to produce 160-bit hashes.
     */
    public static HashingSink hmacSha1(Sink sink, ByteString key) {
        return new HashingSink(sink, key, "HmacSHA1");
    }

    /**
     * Returns a sink that uses the SHA-256 HMAC algorithm to produce 256-bit hashes.
     */
    public static HashingSink hmacSha256(Sink sink, ByteString key) {
        return new HashingSink(sink, key, "HmacSHA256");
    }

    /**
     * Returns a sink that uses the SHA-512 HMAC algorithm to produce 512-bit hashes.
     */
    public static HashingSink hmacSha512(Sink sink, ByteString key) {
        return new HashingSink(sink, key, "HmacSHA512");
    }

    @Override
    public void write(Buffer source, long byteCount) throws IOException {
        checkOffsetAndCount(source.size, 0, byteCount);

        // Hash byteCount bytes from the prefix of source.
        long hashedCount = 0;
        for (Segment s = source.head; hashedCount < byteCount; s = s.next) {
            int toHash = (int) Math.min(byteCount - hashedCount, s.limit - s.pos);
            if (messageDigest != null) {
                messageDigest.update(s.data, s.pos, toHash);
            } else {
                mac.update(s.data, s.pos, toHash);
            }
            hashedCount += toHash;
        }

        // Write those bytes to the sink.
        super.write(source, byteCount);
    }

    /**
     * Returns the hash of the bytes accepted thus far and resets the internal state of this sink.
     *
     * <p><strong>Warning:</strong> This method is not idempotent. Each time this method is called
     * its internal state is cleared. This starts a new hash with zero bytes accepted.
     */
    public final ByteString hash() {
        byte[] result = messageDigest != null ? messageDigest.digest() : mac.doFinal();
        return ByteString.of(result);
    }
}
